1..1 -- A required field. This data element will be available for each record. An example of this would be that each flight record is always assigned a flightId.
0..1 -- An optional field. If the data element is made available to us by our data sources, our system will pass it along to our customers. An example of this would be that if we receive an arrival baggage carousel number assignment, our system will pass this along.
0..n -- An optional field that may repeat multiple times. An example of this would be changes to a FlightHistory record. There may be none when the record is first created but there may be many by the time a flight completes.
1..n -- A mandatory field that may repeat multiple times. An example of this would be positions in a flight track. There will always be at least one (or we would not return the track) but there may be many.
More information in the response element documentation: